Mybatis 源码(二)-三种执行器Executor 的区别

　　mybatis有三种执行器。下面研究其主要区别。

其类图如下：

1. Executor 源码

org.apache.ibatis.executor.Executor：

package org.apache.ibatis.executor;

import java.sql.SQLException;
import java.util.List;

import org.apache.ibatis.cache.CacheKey;
import org.apache.ibatis.cursor.Cursor;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.reflection.MetaObject;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import org.apache.ibatis.transaction.Transaction;

public interface Executor {

  ResultHandler NO_RESULT_HANDLER = null;

  int update(MappedStatement ms, Object parameter) throws SQLException;

  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;

  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;

  <E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException;

  List<BatchResult> flushStatements() throws SQLException;

  void commit(boolean required) throws SQLException;

  void rollback(boolean required) throws SQLException;

  CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql);

  boolean isCached(MappedStatement ms, CacheKey key);

  void clearLocalCache();

  void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType);

  Transaction getTransaction();

  void close(boolean forceRollback);

  boolean isClosed();

  void setExecutorWrapper(Executor executor);

}

2. BaseExecutor 抽象类

public abstract class BaseExecutor implements Executor {

  private static final Log log = LogFactory.getLog(BaseExecutor.class);

  protected Transaction transaction;
  protected Executor wrapper;

  protected ConcurrentLinkedQueue<DeferredLoad> deferredLoads;
  protected PerpetualCache localCache;
  protected PerpetualCache localOutputParameterCache;
  protected Configuration configuration;

  protected int queryStack;
  private boolean closed;

  protected BaseExecutor(Configuration configuration, Transaction transaction) {
    this.transaction = transaction;
    this.deferredLoads = new ConcurrentLinkedQueue<>();
    this.localCache = new PerpetualCache("LocalCache");
    this.localOutputParameterCache = new PerpetualCache("LocalOutputParameterCache");
    this.closed = false;
    this.configuration = configuration;
    this.wrapper = this;
  }

  @Override
  public Transaction getTransaction() {
    if (closed) {
      throw new ExecutorException("Executor was closed.");
    }
    return transaction;
  }

  @Override
  public void close(boolean forceRollback) {
    try {
      try {
        rollback(forceRollback);
      } finally {
        if (transaction != null) {
          transaction.close();
        }
      }
    } catch (SQLException e) {
      // Ignore. There's nothing that can be done at this point.
      log.warn("Unexpected exception on closing transaction.  Cause: " + e);
    } finally {
      transaction = null;
      deferredLoads = null;
      localCache = null;
      localOutputParameterCache = null;
      closed = true;
    }
  }

  @Override
  public boolean isClosed() {
    return closed;
  }

  @Override
  public int update(MappedStatement ms, Object parameter) throws SQLException {
    ErrorContext.instance().resource(ms.getResource()).activity("executing an update").object(ms.getId());
    if (closed) {
      throw new ExecutorException("Executor was closed.");
    }
    clearLocalCache();
    return doUpdate(ms, parameter);
  }

  @Override
  public List<BatchResult> flushStatements() throws SQLException {
    return flushStatements(false);
  }

  public List<BatchResult> flushStatements(boolean isRollBack) throws SQLException {
    if (closed) {
      throw new ExecutorException("Executor was closed.");
    }
    return doFlushStatements(isRollBack);
  }

  @Override
  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    BoundSql boundSql = ms.getBoundSql(parameter);
    CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
    return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
  }

  @SuppressWarnings("unchecked")
  @Override
  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
    if (closed) {
      throw new ExecutorException("Executor was closed.");
    }
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
      clearLocalCache();
    }
    List<E> list;
    try {
      queryStack++;
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }
    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {
        deferredLoad.load();
      }
      // issue #601
      deferredLoads.clear();
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        // issue #482
        clearLocalCache();
      }
    }
    return list;
  }

  @Override
  public <E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException {
    BoundSql boundSql = ms.getBoundSql(parameter);
    return doQueryCursor(ms, parameter, rowBounds, boundSql);
  }

  @Override
  public void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType) {
    if (closed) {
      throw new ExecutorException("Executor was closed.");
    }
    DeferredLoad deferredLoad = new DeferredLoad(resultObject, property, key, localCache, configuration, targetType);
    if (deferredLoad.canLoad()) {
      deferredLoad.load();
    } else {
      deferredLoads.add(new DeferredLoad(resultObject, property, key, localCache, configuration, targetType));
    }
  }

  @Override
  public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
    if (closed) {
      throw new ExecutorException("Executor was closed.");
    }
    CacheKey cacheKey = new CacheKey();
    cacheKey.update(ms.getId());
    cacheKey.update(rowBounds.getOffset());
    cacheKey.update(rowBounds.getLimit());
    cacheKey.update(boundSql.getSql());
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();
    // mimic DefaultParameterHandler logic
    for (ParameterMapping parameterMapping : parameterMappings) {
      if (parameterMapping.getMode() != ParameterMode.OUT) {
        Object value;
        String propertyName = parameterMapping.getProperty();
        if (boundSql.hasAdditionalParameter(propertyName)) {
          value = boundSql.getAdditionalParameter(propertyName);
        } else if (parameterObject == null) {
          value = null;
        } else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
          value = parameterObject;
        } else {
          MetaObject metaObject = configuration.newMetaObject(parameterObject);
          value = metaObject.getValue(propertyName);
        }
        cacheKey.update(value);
      }
    }
    if (configuration.getEnvironment() != null) {
      // issue #176
      cacheKey.update(configuration.getEnvironment().getId());
    }
    return cacheKey;
  }

  @Override
  public boolean isCached(MappedStatement ms, CacheKey key) {
    return localCache.getObject(key) != null;
  }

  @Override
  public void commit(boolean required) throws SQLException {
    if (closed) {
      throw new ExecutorException("Cannot commit, transaction is already closed");
    }
    clearLocalCache();
    flushStatements();
    if (required) {
      transaction.commit();
    }
  }

  @Override
  public void rollback(boolean required) throws SQLException {
    if (!closed) {
      try {
        clearLocalCache();
        flushStatements(true);
      } finally {
        if (required) {
          transaction.rollback();
        }
      }
    }
  }

  @Override
  public void clearLocalCache() {
    if (!closed) {
      localCache.clear();
      localOutputParameterCache.clear();
    }
  }

  protected abstract int doUpdate(MappedStatement ms, Object parameter) throws SQLException;

  protected abstract List<BatchResult> doFlushStatements(boolean isRollback) throws SQLException;

  protected abstract <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql)
      throws SQLException;

  protected abstract <E> Cursor<E> doQueryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds, BoundSql boundSql)
      throws SQLException;

  protected void closeStatement(Statement statement) {
    if (statement != null) {
      try {
        statement.close();
      } catch (SQLException e) {
        // ignore
      }
    }
  }

  /**
   * Apply a transaction timeout.
   *
   * @param statement
   *          a current statement
   * @throws SQLException
   *           if a database access error occurs, this method is called on a closed <code>Statement</code>
   * @since 3.4.0
   * @see StatementUtil#applyTransactionTimeout(Statement, Integer, Integer)
   */
  protected void applyTransactionTimeout(Statement statement) throws SQLException {
    StatementUtil.applyTransactionTimeout(statement, statement.getQueryTimeout(), transaction.getTimeout());
  }

  private void handleLocallyCachedOutputParameters(MappedStatement ms, CacheKey key, Object parameter, BoundSql boundSql) {
    if (ms.getStatementType() == StatementType.CALLABLE) {
      final Object cachedParameter = localOutputParameterCache.getObject(key);
      if (cachedParameter != null && parameter != null) {
        final MetaObject metaCachedParameter = configuration.newMetaObject(cachedParameter);
        final MetaObject metaParameter = configuration.newMetaObject(parameter);
        for (ParameterMapping parameterMapping : boundSql.getParameterMappings()) {
          if (parameterMapping.getMode() != ParameterMode.IN) {
            final String parameterName = parameterMapping.getProperty();
            final Object cachedValue = metaCachedParameter.getValue(parameterName);
            metaParameter.setValue(parameterName, cachedValue);
          }
        }
      }
    }
  }

  private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    List<E> list;
    localCache.putObject(key, EXECUTION_PLACEHOLDER);
    try { list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      localCache.removeObject(key);
    }
    localCache.putObject(key, list);
    if (ms.getStatementType() == StatementType.CALLABLE) {
      localOutputParameterCache.putObject(key, parameter);
    }
    return list;
  }

  protected Connection getConnection(Log statementLog) throws SQLException {
    Connection connection = transaction.getConnection();
    if (statementLog.isDebugEnabled()) {
      return ConnectionLogger.newInstance(connection, statementLog, queryStack);
    } else {
      return connection;
    }
  }

  @Override
  public void setExecutorWrapper(Executor wrapper) {
    this.wrapper = wrapper;
  }

  private static class DeferredLoad {

    private final MetaObject resultObject;
    private final String property;
    private final Class<?> targetType;
    private final CacheKey key;
    private final PerpetualCache localCache;
    private final ObjectFactory objectFactory;
    private final ResultExtractor resultExtractor;

    // issue #781
    public DeferredLoad(MetaObject resultObject,
                        String property,
                        CacheKey key,
                        PerpetualCache localCache,
                        Configuration configuration,
                        Class<?> targetType) {
      this.resultObject = resultObject;
      this.property = property;
      this.key = key;
      this.localCache = localCache;
      this.objectFactory = configuration.getObjectFactory();
      this.resultExtractor = new ResultExtractor(configuration, objectFactory);
      this.targetType = targetType;
    }

    public boolean canLoad() {
      return localCache.getObject(key) != null && localCache.getObject(key) != EXECUTION_PLACEHOLDER;
    }

    public void load() {
      @SuppressWarnings("unchecked")
      // we suppose we get back a List
      List<Object> list = (List<Object>) localCache.getObject(key);
      Object value = resultExtractor.extractObjectFromList(list, targetType);
      resultObject.setValue(property, value);
    }

  }

}

　　可以看到是模板模式的一种设计。其暴露了一些doXXX 方法供下面的具体的Executor 类实现。

3. 具体的Executor

1. org.apache.ibatis.executor.SimpleExecutor 具体的Executor， 每次doQuery、doUpdate 都会创建Statement 对象。默认也是这个Executor

package org.apache.ibatis.executor;

import java.sql.Connection;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.Collections;
import java.util.List;

import org.apache.ibatis.cursor.Cursor;
import org.apache.ibatis.executor.statement.StatementHandler;
import org.apache.ibatis.logging.Log;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import org.apache.ibatis.transaction.Transaction;

public class SimpleExecutor extends BaseExecutor {

  public SimpleExecutor(Configuration configuration, Transaction transaction) {
    super(configuration, transaction);
  }

  @Override
  public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.update(stmt);
    } finally {
      closeStatement(stmt);
    }
  }

  @Override
  public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

  @Override
  protected <E> Cursor<E> doQueryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds, BoundSql boundSql) throws SQLException {
    Configuration configuration = ms.getConfiguration();
    StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, null, boundSql);
    Statement stmt = prepareStatement(handler, ms.getStatementLog());
    Cursor<E> cursor = handler.queryCursor(stmt);
    stmt.closeOnCompletion();
    return cursor;
  }

  @Override
  public List<BatchResult> doFlushStatements(boolean isRollback) {
    return Collections.emptyList();
  }

  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection, transaction.getTimeout());
    handler.parameterize(stmt);
    return stmt;
  }

}

2. org.apache.ibatis.executor.ReuseExecutor 可重用的Executor， 也就是会缓存Statement 对象，重复使用statement。

package org.apache.ibatis.executor;

import java.sql.Connection;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import org.apache.ibatis.cursor.Cursor;
import org.apache.ibatis.executor.statement.StatementHandler;
import org.apache.ibatis.logging.Log;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import org.apache.ibatis.transaction.Transaction;

public class ReuseExecutor extends BaseExecutor {

  private final Map<String, Statement> statementMap = new HashMap<>();

  public ReuseExecutor(Configuration configuration, Transaction transaction) {
    super(configuration, transaction);
  }

  @Override
  public int doUpdate(MappedStatement ms, Object parameter) throws SQLException {
    Configuration configuration = ms.getConfiguration();
    StatementHandler handler = configuration.newStatementHandler(this, ms, parameter, RowBounds.DEFAULT, null, null);
    Statement stmt = prepareStatement(handler, ms.getStatementLog());
    return handler.update(stmt);
  }

  @Override
  public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Configuration configuration = ms.getConfiguration();
    StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
    Statement stmt = prepareStatement(handler, ms.getStatementLog());
    return handler.query(stmt, resultHandler);
  }

  @Override
  protected <E> Cursor<E> doQueryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds, BoundSql boundSql) throws SQLException {
    Configuration configuration = ms.getConfiguration();
    StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, null, boundSql);
    Statement stmt = prepareStatement(handler, ms.getStatementLog());
    return handler.queryCursor(stmt);
  }

  @Override
  public List<BatchResult> doFlushStatements(boolean isRollback) {
    for (Statement stmt : statementMap.values()) {
      closeStatement(stmt);
    }
    statementMap.clear();
    return Collections.emptyList();
  }

  private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    BoundSql boundSql = handler.getBoundSql();
    String sql = boundSql.getSql();
    if (hasStatementFor(sql)) {
      stmt = getStatement(sql);
      applyTransactionTimeout(stmt);
    } else {
      Connection connection = getConnection(statementLog);
      stmt = handler.prepare(connection, transaction.getTimeout());
      putStatement(sql, stmt);
    }
    handler.parameterize(stmt);
    return stmt;
  }

  private boolean hasStatementFor(String sql) {
    try {
      Statement statement = statementMap.get(sql);
      return statement != null && !statement.getConnection().isClosed();
    } catch (SQLException e) {
      return false;
    }
  }

  private Statement getStatement(String s) {
    return statementMap.get(s);
  }

  private void putStatement(String sql, Statement stmt) {
    statementMap.put(sql, stmt);
  }

}

　　可以看到getStatement 方法会以sql为key，去缓存先拿statement 对象。

3. org.apache.ibatis.executor.BatchExecutor

　　可以执行批量update，也就是可以执行多条语句。可以批量修改、插入，也就是使用preparedStatement 对象，后面传入多个参数。

package org.apache.ibatis.executor;

import java.sql.BatchUpdateException;
import java.sql.Connection;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

import org.apache.ibatis.cursor.Cursor;
import org.apache.ibatis.executor.keygen.Jdbc3KeyGenerator;
import org.apache.ibatis.executor.keygen.KeyGenerator;
import org.apache.ibatis.executor.keygen.NoKeyGenerator;
import org.apache.ibatis.executor.statement.StatementHandler;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import org.apache.ibatis.transaction.Transaction;

public class BatchExecutor extends BaseExecutor {

  public static final int BATCH_UPDATE_RETURN_VALUE = Integer.MIN_VALUE + 1002;

  private final List<Statement> statementList = new ArrayList<>();
  private final List<BatchResult> batchResultList = new ArrayList<>();
  private String currentSql;
  private MappedStatement currentStatement;

  public BatchExecutor(Configuration configuration, Transaction transaction) {
    super(configuration, transaction);
  }

  @Override
  public int doUpdate(MappedStatement ms, Object parameterObject) throws SQLException {
    final Configuration configuration = ms.getConfiguration();
    final StatementHandler handler = configuration.newStatementHandler(this, ms, parameterObject, RowBounds.DEFAULT, null, null);
    final BoundSql boundSql = handler.getBoundSql();
    final String sql = boundSql.getSql();
    final Statement stmt;
    if (sql.equals(currentSql) && ms.equals(currentStatement)) {
      int last = statementList.size() - 1;
      stmt = statementList.get(last);
      applyTransactionTimeout(stmt);
      handler.parameterize(stmt);// fix Issues 322
      BatchResult batchResult = batchResultList.get(last);
      batchResult.addParameterObject(parameterObject);
    } else {
      Connection connection = getConnection(ms.getStatementLog());
      stmt = handler.prepare(connection, transaction.getTimeout());
      handler.parameterize(stmt);    // fix Issues 322
      currentSql = sql;
      currentStatement = ms;
      statementList.add(stmt);
      batchResultList.add(new BatchResult(ms, sql, parameterObject));
    }
    handler.batch(stmt);
    return BATCH_UPDATE_RETURN_VALUE;
  }

  @Override
  public <E> List<E> doQuery(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql)
      throws SQLException {
    Statement stmt = null;
    try {
      flushStatements();
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameterObject, rowBounds, resultHandler, boundSql);
      Connection connection = getConnection(ms.getStatementLog());
      stmt = handler.prepare(connection, transaction.getTimeout());
      handler.parameterize(stmt);
      return handler.query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

  @Override
  protected <E> Cursor<E> doQueryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds, BoundSql boundSql) throws SQLException {
    flushStatements();
    Configuration configuration = ms.getConfiguration();
    StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, null, boundSql);
    Connection connection = getConnection(ms.getStatementLog());
    Statement stmt = handler.prepare(connection, transaction.getTimeout());
    handler.parameterize(stmt);
    Cursor<E> cursor = handler.queryCursor(stmt);
    stmt.closeOnCompletion();
    return cursor;
  }

  @Override
  public List<BatchResult> doFlushStatements(boolean isRollback) throws SQLException {
    try {
      List<BatchResult> results = new ArrayList<>();
      if (isRollback) {
        return Collections.emptyList();
      }
      for (int i = 0, n = statementList.size(); i < n; i++) {
        Statement stmt = statementList.get(i);
        applyTransactionTimeout(stmt);
        BatchResult batchResult = batchResultList.get(i);
        try {
          batchResult.setUpdateCounts(stmt.executeBatch());
          MappedStatement ms = batchResult.getMappedStatement();
          List<Object> parameterObjects = batchResult.getParameterObjects();
          KeyGenerator keyGenerator = ms.getKeyGenerator();
          if (Jdbc3KeyGenerator.class.equals(keyGenerator.getClass())) {
            Jdbc3KeyGenerator jdbc3KeyGenerator = (Jdbc3KeyGenerator) keyGenerator;
            jdbc3KeyGenerator.processBatch(ms, stmt, parameterObjects);
          } else if (!NoKeyGenerator.class.equals(keyGenerator.getClass())) { //issue #141
            for (Object parameter : parameterObjects) {
              keyGenerator.processAfter(this, ms, stmt, parameter);
            }
          }
          // Close statement to close cursor #1109
          closeStatement(stmt);
        } catch (BatchUpdateException e) {
          StringBuilder message = new StringBuilder();
          message.append(batchResult.getMappedStatement().getId())
              .append(" (batch index #")
              .append(i + 1)
              .append(")")
              .append(" failed.");
          if (i > 0) {
            message.append(" ")
                .append(i)
                .append(" prior sub executor(s) completed successfully, but will be rolled back.");
          }
          throw new BatchExecutorException(message.toString(), e, results, batchResult);
        }
        results.add(batchResult);
      }
      return results;
    } finally {
      for (Statement stmt : statementList) {
        closeStatement(stmt);
      }
      currentSql = null;
      statementList.clear();
      batchResultList.clear();
    }
  }

}

类似于JDBC提供的批处理，其优缺点如下：

优点：发送的是预编译后的SQL语句，执行效率高。

缺点：只能应用在SQL语句相同，但参数不同的批处理中。因此此种形式的批处理经常用于在同一个表中批量插入数据，或批量更新表的数据。

4. 测试

1. mybatis-config.xml 修改ExecutorType类型为BATCH

  <settings>
    <setting name="defaultExecutorType" value="BATCH"/>
  </settings>

2. 解析原理：

在构造SqlSessionFactory 过程中会调用 org.apache.ibatis.builder.xml.XMLConfigBuilder#parseConfiguration：

  private void parseConfiguration(XNode root) {
    try {
      // issue #117 read properties first
      propertiesElement(root.evalNode("properties"));
      Properties settings = settingsAsProperties(root.evalNode("settings"));
      loadCustomVfs(settings);
      loadCustomLogImpl(settings);
      typeAliasesElement(root.evalNode("typeAliases"));
      pluginElement(root.evalNode("plugins"));
      objectFactoryElement(root.evalNode("objectFactory"));
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      reflectorFactoryElement(root.evalNode("reflectorFactory"));
      settingsElement(settings);
      // read it after objectFactory and objectWrapperFactory issue #631
      environmentsElement(root.evalNode("environments"));
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      typeHandlerElement(root.evalNode("typeHandlers"));
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

1》 settingsAsProperties 解析settings为properties 属性

2》org.apache.ibatis.builder.xml.XMLConfigBuilder#settingsElement 解析 props 属性， 可以看到有一些全局的配置信息，后面会根据这些信息进行初始化

  private void settingsElement(Properties props) {
    configuration.setAutoMappingBehavior(AutoMappingBehavior.valueOf(props.getProperty("autoMappingBehavior", "PARTIAL")));
    configuration.setAutoMappingUnknownColumnBehavior(AutoMappingUnknownColumnBehavior.valueOf(props.getProperty("autoMappingUnknownColumnBehavior", "NONE")));
    configuration.setCacheEnabled(booleanValueOf(props.getProperty("cacheEnabled"), true));
    configuration.setProxyFactory((ProxyFactory) createInstance(props.getProperty("proxyFactory")));
    configuration.setLazyLoadingEnabled(booleanValueOf(props.getProperty("lazyLoadingEnabled"), false));
    configuration.setAggressiveLazyLoading(booleanValueOf(props.getProperty("aggressiveLazyLoading"), false));
    configuration.setMultipleResultSetsEnabled(booleanValueOf(props.getProperty("multipleResultSetsEnabled"), true));
    configuration.setUseColumnLabel(booleanValueOf(props.getProperty("useColumnLabel"), true));
    configuration.setUseGeneratedKeys(booleanValueOf(props.getProperty("useGeneratedKeys"), false));
    configuration.setDefaultExecutorType(ExecutorType.valueOf(props.getProperty("defaultExecutorType", "SIMPLE")));
    configuration.setDefaultStatementTimeout(integerValueOf(props.getProperty("defaultStatementTimeout"), null));
    configuration.setDefaultFetchSize(integerValueOf(props.getProperty("defaultFetchSize"), null));
    configuration.setDefaultResultSetType(resolveResultSetType(props.getProperty("defaultResultSetType")));
    configuration.setMapUnderscoreToCamelCase(booleanValueOf(props.getProperty("mapUnderscoreToCamelCase"), false));
    configuration.setSafeRowBoundsEnabled(booleanValueOf(props.getProperty("safeRowBoundsEnabled"), false));
    configuration.setLocalCacheScope(LocalCacheScope.valueOf(props.getProperty("localCacheScope", "SESSION")));
    configuration.setJdbcTypeForNull(JdbcType.valueOf(props.getProperty("jdbcTypeForNull", "OTHER")));
    configuration.setLazyLoadTriggerMethods(stringSetValueOf(props.getProperty("lazyLoadTriggerMethods"), "equals,clone,hashCode,toString"));
    configuration.setSafeResultHandlerEnabled(booleanValueOf(props.getProperty("safeResultHandlerEnabled"), true));
    configuration.setDefaultScriptingLanguage(resolveClass(props.getProperty("defaultScriptingLanguage")));
    configuration.setDefaultEnumTypeHandler(resolveClass(props.getProperty("defaultEnumTypeHandler")));
    configuration.setCallSettersOnNulls(booleanValueOf(props.getProperty("callSettersOnNulls"), false));
    configuration.setUseActualParamName(booleanValueOf(props.getProperty("useActualParamName"), true));
    configuration.setReturnInstanceForEmptyRow(booleanValueOf(props.getProperty("returnInstanceForEmptyRow"), false));
    configuration.setLogPrefix(props.getProperty("logPrefix"));
    configuration.setConfigurationFactory(resolveClass(props.getProperty("configurationFactory")));
    configuration.setShrinkWhitespacesInSql(booleanValueOf(props.getProperty("shrinkWhitespacesInSql"), false));
    configuration.setDefaultSqlProviderType(resolveClass(props.getProperty("defaultSqlProviderType")));
  }

3. 测试修改：

  @Test
  void updateProjectTest() {
    try (SqlSession sqlSession = sqlSessionFactory.openSession()) {
      final AutoConstructorMapper mapper = sqlSession.getMapper(AutoConstructorMapper.class);
      mapper.updateSubject(1, "1");
      System.out.println("111222");
      mapper.updateSubject(1, "2");
    }
  }

查看日志如下：

DEBUG [main] - ==>  Preparing: update subject set name = ? WHERE id = ?
DEBUG [main] - ==> Parameters: 1(Integer), 1(String)
111222
DEBUG [main] - ==> Parameters: 1(Integer), 2(String)
DEBUG [main] - Rolling back JDBC Connection [com.mysql.jdbc.JDBC4Connection@3a3e78f]
DEBUG [main] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@3a3e78f]
DEBUG [main] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@3a3e78f]

　　可以看到发了一次SQL

4. 使用默认的 SimpleExecutor 查看默认的SQL如下：

DEBUG [main] - ==>  Preparing: update subject set name = ? WHERE id = ?
DEBUG [main] - ==> Parameters: 1(Integer), 1(String)
DEBUG [main] - <==    Updates: 1
111222
DEBUG [main] - ==>  Preparing: update subject set name = ? WHERE id = ?
DEBUG [main] - ==> Parameters: 1(Integer), 2(String)
DEBUG [main] - <==    Updates: 2

5. CachingExecutor

　　缓存执行器。是一个Executor的装饰器。它为Executor对象增加了二级缓存的相关功能，委托的执行器对象可以是SimpleExecutor、ReuseExecutor、BatchExecutor中任一一个。

　　执行 update 方法前判断是否清空二级缓存；执行 query 方法前先在二级缓存中查询，命中失败在通过被代理类查询。

package org.apache.ibatis.executor;

import java.sql.SQLException;
import java.util.List;

import org.apache.ibatis.cache.Cache;
import org.apache.ibatis.cache.CacheKey;
import org.apache.ibatis.cache.TransactionalCacheManager;
import org.apache.ibatis.cursor.Cursor;
import org.apache.ibatis.mapping.BoundSql;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.mapping.ParameterMapping;
import org.apache.ibatis.mapping.ParameterMode;
import org.apache.ibatis.mapping.StatementType;
import org.apache.ibatis.reflection.MetaObject;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import org.apache.ibatis.transaction.Transaction;

public class CachingExecutor implements Executor {

  private final Executor delegate;
  private final TransactionalCacheManager tcm = new TransactionalCacheManager();

  public CachingExecutor(Executor delegate) {
    this.delegate = delegate;
    delegate.setExecutorWrapper(this);
  }

  @Override
  public Transaction getTransaction() {
    return delegate.getTransaction();
  }

  @Override
  public void close(boolean forceRollback) {
    try {
      // issues #499, #524 and #573
      if (forceRollback) {
        tcm.rollback();
      } else {
        tcm.commit();
      }
    } finally {
      delegate.close(forceRollback);
    }
  }

  @Override
  public boolean isClosed() {
    return delegate.isClosed();
  }

  @Override
  public int update(MappedStatement ms, Object parameterObject) throws SQLException {
    flushCacheIfRequired(ms);
    return delegate.update(ms, parameterObject);
  }

  @Override
  public <E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException {
    flushCacheIfRequired(ms);
    return delegate.queryCursor(ms, parameter, rowBounds);
  }

  @Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    BoundSql boundSql = ms.getBoundSql(parameterObject);
    CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
    return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

  @Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
      throws SQLException {
    Cache cache = ms.getCache();
    if (cache != null) {
      flushCacheIfRequired(ms);
      if (ms.isUseCache() && resultHandler == null) {
        ensureNoOutParams(ms, boundSql);
        @SuppressWarnings("unchecked")
        List<E> list = (List<E>) tcm.getObject(cache, key);
        if (list == null) {
          list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
          tcm.putObject(cache, key, list); // issue #578 and #116
        }
        return list;
      }
    }
    return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

  @Override
  public List<BatchResult> flushStatements() throws SQLException {
    return delegate.flushStatements();
  }

  @Override
  public void commit(boolean required) throws SQLException {
    delegate.commit(required);
    tcm.commit();
  }

  @Override
  public void rollback(boolean required) throws SQLException {
    try {
      delegate.rollback(required);
    } finally {
      if (required) {
        tcm.rollback();
      }
    }
  }

  private void ensureNoOutParams(MappedStatement ms, BoundSql boundSql) {
    if (ms.getStatementType() == StatementType.CALLABLE) {
      for (ParameterMapping parameterMapping : boundSql.getParameterMappings()) {
        if (parameterMapping.getMode() != ParameterMode.IN) {
          throw new ExecutorException("Caching stored procedures with OUT params is not supported.  Please configure useCache=false in " + ms.getId() + " statement.");
        }
      }
    }
  }

  @Override
  public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
    return delegate.createCacheKey(ms, parameterObject, rowBounds, boundSql);
  }

  @Override
  public boolean isCached(MappedStatement ms, CacheKey key) {
    return delegate.isCached(ms, key);
  }

  @Override
  public void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType) {
    delegate.deferLoad(ms, resultObject, property, key, targetType);
  }

  @Override
  public void clearLocalCache() {
    delegate.clearLocalCache();
  }

  private void flushCacheIfRequired(MappedStatement ms) {
    Cache cache = ms.getCache();
    if (cache != null && ms.isFlushCacheRequired()) {
      tcm.clear(cache);
    }
  }

  @Override
  public void setExecutorWrapper(Executor executor) {
    throw new UnsupportedOperationException("This method should not be called");
  }

}

 　　而且默认SqlSession 中的Executor 是CachingExecutor， CachingExecutor 内部维护的代理delegate是 SimpleExecutor。